And some of the heavier parts of the oil are on the bottom of the Kalamazoo River.

This is the first time the U.S. Environmental Protection Agency has had a diluted bitumen spill of this size, and responders are “writing the book” on how to respond, said Ralph Dollhopf, EPA incident commander for the spill.

“At minimum, we’re writing a chapter in the oil spill cleanup book on how to identify submerged oil,” Dollhopf said. “We’re writing chapters on how it behaves once it does spill (and) how to recover it.”

What the EPA didn’t expect at the beginning of the spill last July was how much time they would spend extracting the heavier oil submerged in the bottom of the Kalamazoo River.

“In a situation where we don’t have to be concerned with submerged oil, then we clean up the oil on the surface and be done,” Dollhopf said.

But for this spill on a 35-mile stretch of the Kalamazoo River and Talmadge Creek, between 60 and 70 percent of the cleanup time has concerned the submerged oil that sank to the bot­tom of the river and mixed with the sediment, he said.

The EPA has never responded to a spill like this before, Dollhopf said.

The EPA, Michigan Department of Environmental Quality, Calhoun and Kalamazoo county governments, Enbridge and their contractors had to come up with new strategies and techniques to collect the oil.

Jonathon Gruenke / Kalamazoo GazetteOil on the surface of the Kalamazoo River near Ceresco Dam in Marshall. Cleanup crews work in the Kalamazoo River Friday afternoon to remove oil from the 2010 spill.

According to the EPA's website, 766,000 gallons have been recovered and most was floating at the top of the river. Dollhopf said it is unknown how much oil sank to the bottom.

The long-term impact on wildlife and the environment also is unknown because there has never been an inland spill of diluted bitumen of this magnitude. Scientists have said they have little frame of reference for comparison.

The DEQ is currently conducting a study into the possibility of health effects of the submerged oil if humans come in contact with it. This is one of the major factors deciding when the river will be open to the public.

Besides providing a new challenge for the EPA and environmentalists, the oil pumped through the pipeline is at the center of a discussion over the future of energy in the country.

Keystone XL, a proposed 1,661-mile pipeline that would run from Alberta, Canada, to Texas, is drawing in­creased attention now as politicians, environmentalists and business advocates debate the construction of new diluted bitumen pipelines in the United States — as well as the safety of current pipelines.

Both are a mixture of bitumen, which is a heavy, asphaltic material, as well as a dilutant, a lighter component which easily evaporates. The dilutant is a natural gas condensate and includes benzene, toluene, and hydrogen sulfide.

The bitumen is almost solid at room temperature. To get it out of the ground, a process called "steam­ assisted gravity drainage" is used, which injects steam into the ground to loosen up the oil to be transported to the surface, according to a report from the House Transportation and Infrastructure Committee. The raw crude is then mixed with chemicals to help move it through the pipeline to refineries.

Because the diluted bitumen is so thick, it is heated so it can pass through the pipeline, said Anthony Swift, energy analyst for the inter­national program at the Natural Re­sources Defense Council.

Oil sands have been transferred out of Alberta via pipeline since the 1980s. In the past 10 years, imports of diluted bitumen to the United States have increased almost sixfold, to 600,000 barrels a day in 2010, Swift said.

When Enbridge's pipeline ruptured last year in Marshall, the lighter part of the oil evaporated, “making the heavy mixture even more heavy as it moved down the creek and down the river; it had an increased tendency to sink,” Dollhopf said. “It’s the nature of the mixture of the oil that caused it to sink.”

The oil is in six inches of the sediment at the bottom of the river, he said.

Dollhopf noted that since this is the only large diluted bitumen spill the EPA has dealt with, his comments pertain to only to the Kalamazoo River.

“My comments go to this particular spill, this particular oil. We don’t have any other frame of reference,” Dollhopf said. “I can’t tell you this is characteristic of tar sands. We don’t know anything about tar sands. We only know about this oil.”

Near the Ceresco Dam on Thursday, boats were going back and forth on a stretch of the river, stirring up the bottom of the riverbed. Booms were floating in the water. Driving east to west, some were pulling marsh excavators to agitate the bottom of the river. Other boats were injecting water or air to stir the bottom.

The EPA issued a directive to Enbridge to have these three areas cleaned up by Aug. 31.

These are three areas on the river where sediments naturally collect, Dollhopf said. As the river flows, and the sediments move, they start to come together in these areas. And, over time, the smaller bits of oil that the EPA couldn’t detect earlier start to accumulate in those areas.

In the spring, the EPA and Enbridge did a full assessment of the river by air, on the shore and in the water. That’s when they discovered the lo­cation of the submerged oil.

Anywhere between 500 to 700 people are working at the scene of the oil spill at any given time, Dollhopf said. There were about 2,500 people at the height of the spill in August.

In April, Enbridge reported spending $550 million on the spill, not including insurance recoveries, fines and penalties. Enbridge spokesman Jason Manshum said the company expects to pay approximately 90 percent of estimated costs this year.

The EPA has spent $29.1 million so far, all of which will be repaid by En­bridge, said Susan Hedman, EPA Region 5 administrator.

Much of these costs, Dollhopf said, are be­cause the EPA and other agencies had to come up with new strategies to handle the submerged oil, and Enbridge’s contractor crews developed new methods to release the oil from the bottom of the river.

“That’s extended the amount of time for the cleanup,” Dollhopf said. “And, without a doubt, that’s increased the cost of the cleanup. That’s really the most significant ramification.”

But that innovation is one success story of the spill, Dollhopf said.

They also are finishing cleanup else­where along the river this summer.

The 200 acres of submerged oil they are targeting this summer make up a small fraction of the total 35 miles affected by the spill, Dollhopf said.

Dollhopf said he expects that, in the fall, when the EPA, DEQ and Enbridge do some reassessment of the river, they will likely find more submerged oil that could not be detected earlier.

“That process is likely to be iterative,” he said. “We just can’t say how many iterations we will go through.”